Variable Valve Activation (VVA) mechanisms for internal combustion engines are well known. It is known to lower the lift, or even to provide no lift at all, of one or more valves of a multiple-cylinder engine, during periods of light engine load. Such deactivation or valve lift switching can substantially improve fuel efficiency.
Various approaches are known for changing the lift of valves in a running engine. One known approach is to provide an intermediate cam follower arrangement, which is rotatable about the engine camshaft and is capable of changing the valve lift and timing, the camshaft typically having both high-lift and low-lift lobes for each such valve.
For example, a Roller Finger Follower (RFF) typically acts between a rotating camshaft lobe and a pivot point such as a Hydraulic Lash Adjuster (HLA) to open and close an engine valve. By way of example, a switchable deactivation RFF includes an outer arm, also known as a body or low-lift follower, and an inner arm, also known as high-lift follower. The inner arm supports a roller carried by a shaft. Alternatively, the outer arm, rather than the inner arm, may support a pair of rollers carried by a shaft. Also alternatively, both the outer and inner arms may support rollers or both the outer an inner arms may not have rollers but rather incorporate sliding surfaces. The roller is engaged by a lobe of an engine camshaft that causes the outer arm to pivot about the HLA, thereby actuating an associated engine valve. The deactivation RFF is selectively switched between a coupled (high-lift) and decoupled (zero-lift) mode. In the coupled mode the inner arm is coupled to the outer arm by a movable latching mechanism and rotation of the lifting cam is transferred from the roller through the shaft to pivotal movement of the outer arm, which in turn, reciprocates the associated valve. In the decoupled mode, the inner arm is decoupled from the outer arm. Thus, the inner arm does not transfer rotation of the lifting cam lobe to pivotal movement of the outer arm, and the associated valve is not reciprocated. In this mode, the roller shaft is reciprocated within the outer arm.
A switchable, two-step RFF operates in a manner similar to the deactivation RFF, as described above. However, one particular difference between the operation of a deactivation RFF and a two-step RFF occurs in the decoupled mode of operation. When in the decoupled (zero-lift) mode, the outer arm of a deactivation RFF may be engaged by zero-lift cam lobes and remains in a static position allowing the associated valve to remain closed. On the other hand, when in decoupled (low-lift) mode, the outer arm of a two-step RFF is engaged by low-lift camshaft lobes to thereby reciprocate the associated engine valve according to the lift profile of the low-lift camshaft lobe.
A lost motion spring maintains contact between the roller and the lifting portion of the camshaft lobe when either type of RFF (i.e., deactivation or two-step) is in the decoupled (zero-lift or low-lift, respectively) mode and absorbs the reciprocal motion of the shaft and roller. The lost motion spring biases the inner arm away from the outer arm of the RFF. The expansion force of the lost motion spring acting on the inner arm must on the one hand be sufficient to maintain contact of the roller with the lifting portion of the cam lobe, while on the other hand must not cause the HLA, which supports the outer arm to be pumped down by the force of the lost motion spring.
Another known approach is to provide a deactivation mechanism in the Hydraulic Lash Adjuster (HLA) upon which a cam follower rocker arm pivots. Such arrangement is advantageous in that it can provide variable lift from a single cam lobe by making the HLA either competent or incompetent to transfer the motion of the cam eccentric to the valve stem. Yet another known approach is to provide a deactivation mechanism in the Hydraulic Valve Lifter (HVL).
During the operation of the above mentioned two-mode variable valve activation devices the two-mode variable valve actuation device may fail to achieve the selected mode of lift. U.S. Pat. No. 7,761,217 teaches a mechanism for detecting the lift mode by positioning a piezoelectric element relative to the lost motion spring such that the piezoelectric element acts a radio transmitter to transmit a radio signal when the piezoelectric element is subjected to a compressive load of the lost motion spring. In this way, a receiver can receive the radio signal and determine which lift mode the two-mode variable valve activation device is in and make a comparison against the selected lift mode. While this arrangement may be effective, cost and complexity is added to the system by the addition of the piezoelectric element and the receiver needed to receive the signal.
What is needed is a method for diagnosing or determining the lift mode of a two-mode variable valve actuation device.